A cohort of 236 vaginally delivered neonates weighing > or = 4,200 g was evaluated. Shoulder dystocia was encountered in 27 deliveries (11.4%) and brachial plexus injury was diagnosed in 3 infants (1.3%). The 'traditional' antepartum risk factors could not be associated with shoulder dystocia. In this cohort, primiparity was significantly more frequent among the dystocia cases (OR = 8.58, 99% CI = 1.35-54.35, p = 0.021). Shoulder dystocia could not be attributed to a particular difference between the current and the previous heaviest birth weight. A policy of cesarean section for all infants weighing > or = 4,200 g would result in at least 5- to 6-fold increase in cesarean rate in this group of patients. Our data reconfirm that shoulder dystocia and brachial plexus injury are unpredictable, even in macrosomic infants. It is a matter of policy whether to accept the expected 1:9 and 1:79 respective risks associated with vaginal births.